**4. Conclusions**

In this work, a FORTRAN code based on a hybrid method (which uses a combination of LBM, SPM, and FDM) has been developed to simulate the natural convection inside an annulus between two concentric hexagonal cylinders. After validating the numerical code by applying it to a standard benchmarking problem, natural convection simulations inside the hexagonal annulus have been performed by considering different combinations of Rayleigh number, Ra, and aspect ratio, *AR*. When *AR* = 0.6, two separate thermal plumes are formed due to the separation of convective flow at the upper corner of inner hexagon, which is in accordance with the previous studies. To verify the accuracy and robustness of the present method for simulating natural convection flow inside the hexagonal annulus, all the simulation results obtained from the present technique have been compared with the Fluent results. The simulation results of isotherms and streamlines patterns, temperature, and velocity distributions inside the annulus show good agreemen<sup>t</sup> with those obtained from Fluent software.

**Funding:** This research was funded by Kyungsung University Research Grants in 2019 and the APC was funded by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (Ministry of Science and ICT) (2020R1G1A1010247).

**Acknowledgments:** This research was supported by Kyungsung University Research Grants in 2019 and this research was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (Ministry of Science and ICT) (2020R1G1A1010247).

**Conflicts of Interest:** The author declares no conflict of interest.
